This invention pertains to a procedure for opening petroleum producing and transporting equipment in a way that reduces the danger of electrostatic sparking inside the equipment and decreases the time required to safely open such equipment.
In petroleum producing and transporting operations, static electricity has proven to be a source of fire hazard. Static electrical charge is generated at the internal surface of coated or uncoated petroleum handling equipment, and the interface of the solid, liquid and gas phases flowed, agitated, or settled in such equipment. The sign of the charge in the petroleum liquid depends on the materials involved. Some crude oils, for example, are negative while others are positive.
The charge in the liquid within a piece of producing or transporting equipment attracts an equal and opposite charge at the inner surface of the equipment, but the combination will give no external evidence of the charge. It is standard practice to ground all of the equipment to a common ground conductor, but in petroleum producing and transporting equipment, this will not prevent the formation of a hazardous internal electrostatic charge. This sort of electrical grounding works only when both the surface of the equipment and the petroleum liquid are highly conductive, and electrostatic charges are discharged as fast as they are produced. Petroleum producing and transportation operations use equipment with poorly conductive or coated surfaces and use fluids with relatively poor conductivity. Moreover, the fluids used frequently contain mixed phases. Mixed phases are prone to produce a static charge even if water is present as one of the phases.
When the fluid in such equipment is removed or drained so that some work may be done inside the equipment, the charge on the internal surface of the equipment and on petroleum and inorganic deposits in the equipment remains. The natural decline in the charge, in many cases, is excessively slow. When the petroleum materials are drained, the charge, therefore, remains dangerous in such equipment for costly and indeterminate periods. The danger is greatest in cold areas like those found and experienced in Alaska. In order to be safe, it would be necessary to keep such equipment sealed and out of service for a long period, for example, two weeks. If air leaked into such equipment during such waiting period and an explosive ratio of oxygen to flammable gas developed, an explosion could occur as the charge is dissipated.
U.S. Pat. No. 3,728,156 refers to a method of cleaning tanks using a high velocity liquid spray. In the cleaning method, the amount of electrostatic charge created by the spray is controlled by alternating between cleaning liquids that build opposite charges, e.g., alternating between salt water and fresh water. U.S. Pat. No. 3,893,003 refers to a method of reducing an electrostatic charge in a tanker by converting water or salt water to mist and spraying the mist inside the tank. It is said that the mist acquires a charge opposite in polarity to the polarity of the initial electrostatic charge in the tank. It is to be noted that in both of these patents a spray is used and the spray creates an electrostatic charge. This increases the hazards of sparking an existing electrostatic charge, rather than alleviating the hazard. As previously noted, the polarity of the charge inside the petroleum handling equipment varies. In this invention, these problems are overcome by using a continuous or steady stream of liquid designed to allow an existing electrostatic charge to dissipate or leak off without creating an opposite electrostatic charge which increases the hazard.